Electric power generation utilizing small temperature differences is important to the promotion of energy conservation. In this case it is necessary to use a boiling heat exchanger with high performance because, due to the small temperature difference (less than 30.degree. C.) between the heat source and the heat exchange medium, it is necessary to boil the medium by utilization of as small temperature difference as possible even in the case of a heat exchanger. To this end, a metal heat transfer surface with a complexly manufactured surface is used. However, although use of an enhanced boiling surface promotes nucleate boiling heat transfer over that obtainable with a smooth metal surface in the region of small temperature difference, it also causes a number of bubbles to be produced so that shift to film boiling occurs at a low temperature difference. This results in the shifting to film boiling in the vicinity of the inlet of the boiling heat exchanger to deteriorate the heat transfer performance. For this reason, it has been suggested that the transition to film boiling be delayed by applying an electric field.
More specifically, the boiling curve in boiling heat transfer is shown by the curve I in FIG. 1. That is, the curve I moves from the nucleate boiling region a to the peak P.sub.1, maximum boiling heat flux, and when entering the film boiling region, the heat flux Q is abruptly lowered as shown at a'.
The entire quantity of heat transfer is increased in the present invention as shown by the boiling curve II, by promoting and augmenting the heat transfer in the nucleate boiling state and delaying the shift to film boiling. (The peak P.sub.2 is the maximum boiling heat flux.) Although the boiling heat exchanger exhibits the highest temperature difference at its inlet and the lowest temperature difference at its outlet, high heat transfer performance must be caried out over the entire portion of the heat exchanger.
There have been proposed various methods for promoting boiling heat transfer, such as a method utilizing application of an elelctric field to a boiling surface, for example. It was, however, believed that the augmentation of the heat transfer by application of electric fields would merely bring about augmentation of the maximum boiling heat flux. In other words, it has been little known that the effects resulting from the shape of an electrode or a heat transfer surface contribute to augmentation of the heat flux in the nucleate boiling region and no one has taken heat exchange media into consideration.
The aforementioned method utilizing application of an electric field will be described with reference to FIG. 2.
High voltage is applied between a heat transfer surface 3 having its back held in contact with a medium 1 from which heat is to be transferred and electrode 4 in the shape of rods, plates, a net, or the like placed in a heat exchange medium 2, and an electric field is applied to the heat exchange medium 2 in the neighborhood of the heat transfer surface 3.
With this, the boiling curve I of FIG. 1 assumes the curve III and the maximum boiling heat flux is shifted from point P.sub.1 to point P.sub.3, and it is known that the maximum boiling heat flux is two to three times of the case wherein the electric field is not applied.
However, the conventional method for increasing the maximum boiling heat flux by the electric field merely contemplates the application of the high voltage to the heat transfer surface by means of the aforementioned electrodes and does not pay any attention to the optimization of other conditions. One of the reasons is that neither a physical mechanism for determining the maximum boiling heat flux by the electric field nor a theoretical analysis has been accomplished. With no theoretical analysis, it is difficult to obtain the factors for optimization, and there is no choice but to use of the voltage as the only factor.